In general, to coat a solder on a base metal, oxides and other stains on the surface of the metal are removed, and simultaneously, a re-oxidation of the metal surface during the soldering is prevented, to reduce the surface tension of the solder and improve the wettability of the melted solder with the metal surface, and for this purpose, a soldering flux is used.
As the soldering flux, usually a resin flux formed by incorporating an activator capable of removing an oxide film into a thermoplastic resin having a low softening point, such as rosin, is used.
However, where the soldered portion is exposed to a high-temperature atmosphere, for example, to an atmosphere of 80.degree. C. as in an engine room of an automobile, if the flux comprising a thermoplastic resin having a softening point below about 70.degree. C., such as rosin, the thermoplastic resin is melted and an active ion in the activator held in this thermoplastic resin is released to lower the electrically insulating property and cause a corrosion of the metal surface.
This problem has been resolved by washing and removing the residual flux after the soldering, but since flourocarbon or the like is used as the washing solvent, a problem arises of environmental pollution, and furthermore, the cost is increased by the use of the washing solvent and the washing step.
Therefore, a primary object of the present invention is to provide a soldering flux capable of maintaining the electrically insulating property without washing away the residual flux, i.e., without washing, even if used in a high-temperature atmosphere.